The present invention relates to a process for the production of rubber-modified vinyl aromatic copolymers, particularly rubber-modified styrene copolymers having improved surface appearance and impact properties. More particularly, the present invention relates to the copolymerization of vinyl aromatic compounds in the presence of an elastomer in a single suspension copolymerization step.
Two of the most widely known impact polymers are high impact polystyrene, a copolymer comprising styrene monomer and rubber polymer in which particles of grafted rubber are homogeneously dispersed in a continuous phase of polymerized styrene monomer, and ABS polymers, a copolymer comprising styrene monomer, acrylonitrile, and a rubbery polymer in which particles of grafted rubber are homogeneously dispersed in a continuous phase comprising styrene-acrylonitrile copolymer. These polymers have found numerous commercial applications because of their unusual combination of chemical and physical properties, particularly their excellent mechanical properties, such as impact strength. It is well known in the art that high impact polystyrene and ABS polymers can be prepared according to any of the usual styrene monomer polymerization processes, such as mass (bulk), emulsion, or suspension polymerizations. However, such conventional polymerization methods have certain disadvantages associated therewith which renders their use undesirable for the commercial preparation of rubber-modified vinyl aromatic copolymers. In emulsion polymerization, recovery of the copolymer is very difficult, requiring coagulation of the latex, separation of the waste water phase, and drying of the polymer. While suspension polymerization overcomes the recovery problems associated with emulsion polymerization, resins prepared by suspension copolymerization have been found to exhibit poor impact properties as the amount of rubber which can be successfully incorporated therein is too small. It is well known in the art that the essential properties of rubber-modified vinyl aromatic copolymers are dependent on many factors, and are particularly dependent upon the concentration of rubbery polymer, which confers shock absorbing and impact resistance properties to the resin, which can be incorporated therein, and on the degree of dispersion of the rubbery particles in the resin. With conventional suspension polymerization processes, the small amounts of rubbery polymer which can be successfully incorporated into the product resin produces copolymers having less than desirable impact properties and surface appearance. Typical examples of prior art suspension polymerization processes are disclosed in U.S. Pat. Nos. 3,047,534; 3,627,855; and, 3,786,115. Similarly, bulk or mass polymerization processes are limited in the amount of rubber that can be grafted. A variation of the conventional bulk polymerization process is described in U.S. Pat. No. 2,606,163, in which the impact strength, tensile strength, and percent elongation values of styrene copolymers comprising 85 to 98 parts by weight of styrene and from 1 to 15 parts by weight of a rubbery polymer are improved by mechanically working the copolymer, such as by milling the bulk-polymerized copolymer or by mechanically stirring the reaction mixture during the bulk polymerization.
Heretofore, it has been the usual practice in the prior art to produce rubber-modified vinyl aromatic copolymers in a two-step polymerization process comprising a bulk prepolymerization first stage, and a suspension polymerization second stage. In this two-step process, the elastomer or rubber is first dissolved in monomer, such as styrene and mixtures thereof with other monomers copolymerizable therewith, followed by a polymerization in bulk up to a degree of conversion depending on the molecular weight and the concentration of the rubber. The resulting bulk prepolymer is then suspended in water and the polymerization is continued under suspension conditions until complete conversion of the reactants to the rubber-modified copolymer is obtained. Representative of this type of polymerization process for rubber-modified copolymers are the polymerization processes disclosed in U.S. Pat. Nos. 3,428,712; 3,660,534; 3,696,172; and 3,781,383.
While the foregoing two-step polymerization process has been found to be advantageous for the production of rubber-modified copolymers having a low rubber concentration, on a commercial scale this polymerization process has encountered several significant difficulties which preclude the use of this process in the preparation of rubber-modified copolymers having a rubber concentration of greater than approximately 10%. In large industrial reactors, the viscosity of the mass at the time of phase inversion and suspension must not be too high, since a highly viscous mass often exceeds the economically feasible stirring capacity of the reactor, and is very difficult to uniformly disperse into suspension. Accordingly, the commercial production of rubber-modified styrene copolymers by the two-step polymerization process has been limited to the production of copolymers having a rubber concentration of less than about 10% rubber by weight. As a result, the quality of the rubber-modified styrene copolymers obtained by this two-step process are inferior when compared to copolymers prepared by emulsion polymerization, particularly with respect to impact strength, especially low-temperature impact strength, and surface gloss appearance.
There exists a great need in the art, therefore, for a polymerization process for the production of high rubber content styrene copolymers, which eliminates the difficulties and disadvantages of the aforementioned conventional polymerization processes, and enables impact polymers having large rubber concentrations and exhibiting excellent physical properties, such as impact strength and surface appearance, to be produced.